Traditionally, most commercially available inkjet printers have a print engine which forms part of the overall structure and design of the printer. The body of the printer unit is usually constructed to accommodate the printhead and associated media delivery mechanisms, and these features are integral with the printer unit.
This is especially the case with inkjet printers that employ a printhead that traverses back and forth across the media as the media progresses through the printer unit in small iterations. Typically, the reciprocating printhead is mounted to the body of the printer unit such that it can traverse the width of the printer unit between a media input roller and a media output roller, with the media input and output rollers forming part of the structure of the printer unit. It may be possible to remove the printhead for replacement, however the other parts of the print engine, such as the media transport rollers, control circuitry and maintenance stations, are usually fixed within the printer. Replacement of these parts is not possible without replacement of the entire printer.
As well as being rather fixed in their design construction, printers employing reciprocating type printheads are relatively slow, particularly when performing print jobs of full colour and/or photo quality. This is due to the fact that the printhead must continually scan the stationary media to deposit the ink on the surface of the media and it may take a number of swathes of the printhead to deposit one line of the image.
Recently, ‘pagewidth’ printheads have been developed that extend the entire width of the print media. The printhead remains stationary as the media is transported past its array of nozzles. This increases print speeds as the printhead no longer needs to perform a number of swathes to deposit a line of an image. Instead, the printhead deposits the ink on the media as it moves past at high speeds. With these printheads, full colour 1600 dpi printing at speeds of around 60 pages per minute are possible. Such speeds were unattainable with conventional inkjet printers.
It is desirable to lower the pressure of the ink at the nozzles to avoid ink leakage. By keeping the hydrostatic pressure of ink at the nozzle less than atmospheric, the ink meniscus does not bulge outwardly. If the ink meniscuses bulge out of the nozzles, paper dust or other contaminants can break the surface tension and wick the ink onto the nozzle plate. Similarly, if the printhead is jarred during operation, ink is much less likely to be shaken from the nozzles if the ink is at a slightly negative pressure. However, the negative pressure can not be too strong or the nozzle chambers will de-prime of ink. Also, as the ejection actuators must act against any negative pressure, keeping a constant steady state pressure at the nozzles will help to control the drop ejection characteristics.
Unfortunately, as the ink from the ink supply (usually a cartridge) is used by the printhead, the ink level drops and so to does the head of ink above the individual nozzles (or at least the cartridge outlet). Therefore, the hydrostatic pressure changes as the ink is consumed. It is possible to keep the negative ink pressure relatively constant using an ink bag and constant force spring arrangement such as that used in U.S. application Ser. No. 11/014,769 filed Dec. 20, 2004. However, this is not a very efficient use of ink as there is a significant amount of residual in left in the ink bag when it is deemed ‘empty’.